1. Field of the Invention
The present invention relates to an optical film that exhibits a low haze and a high contrast when being assembled into a liquid crystal display device, and method of producing the same. In particular, this invention relates to an optical compensatory film using the optical film, a polarizing film, and a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display device has been widely used as an image display device that is capable of reducing power consumption and size. Conventionally, one of major disadvantages of the liquid crystal display device is high viewing angle dependency of images. However, in recent years, a VA liquid crystal mode with wide viewing angle has been put to practical use, thereby remarkably increasing demand for the liquid crystal display device in a market that requires high quality image such as television. For this reason, it is required for the liquid crystal display device, to improve the optical compensating function of optical compensatory members such as cellulose acylate film that is used for improving color, contrast, and viewing angle dependency.
Generally, the cellulose acylate film is produced by adding inorganic fine particles or an additive into the cellulose acylate solution dope, and then casting them. In this case, in the cellulose acylate solution dope, the inorganic fine particles are generally used as a matting agent so as to prevent the adhesion of film during the winding-up. However, when adding the inorganic fine particles, there are problems in that a haze of the obtained film increases and the black brightness of the liquid crystal display device with the film becomes high during black display, which causes the remarkable deterioration of the brightness. In order to cope with the above problems, a technique that hydrophobized particles are used in a process of dissolving the matting agent in the dope so as to prevent re-aggregation of the dispersed matting agent is disclosed (see JP-A No. 2001-2799). Further, in connection with a phenomenon that the haze of the stretched film increase due to the craze of the cellulose acylate, an additive that has a melting point or glass transition point (Tg) lower than the cellulose acylate is used to increase the concentration of the additive near the particles. Accordingly, it is possible to prevent the generation of craze near the particles, and to reduce the haze (see JP-A No. 2003-301049).
In recent years, it is suggested to manufacture an optical compensatory film having a high retardation value that is capable of being used even when the optical anisotropy is demanded by using a cellulose acylate film. In order to obtain high optical anisotropy, the film is generally added with a hydrophobic retardation regulating agent with high reflective index. Furthermore, in the future, it is required to improve the function of the optical compensatory film by using an additive that functions as an ultraviolet absorber and a moisture permeability regulating agent in the optical compensatory film.
However, when the additive containing the above organic compounds is added to the film, there are problems in that the haze of the optical film increases and the contrast of the liquid crystal display device using the above-mentioned film is remarkably deteriorated. Even though the technologies disclosed in JP-A Nos. 2001-2799 and 2003-301049 are used, it is difficult to satisfactorily suppress the increase in the haze of the optical film and the degradation of the contrast of the liquid crystal display device that uses the film. The above-mentioned problems are remarkable in the case of film to which a hydrophobic retardation regulating agent with high reflective index is added and in the case when high retardation is required.